Stacked-on branch not unlocked when commit_write_group fails on stacked branch

When Repository.commit_write_group fails on a write_locked repo, unlock() will fail to unlock fallback repositories:

This was seen when the failure:

  Module XXX, line YYY, in ZZZ
    branch.pull(source_branch, overwrite=True)
  Module <string>, line 4, in pull_write_locked
  Module bzrlib.branch, line 1931, in pull
    _override_hook_target=_override_hook_target)
  Module <string>, line 4, in pull_write_locked
  Module bzrlib.branch, line 1736, in pull
    graph=graph)
  Module <string>, line 4, in update_revisions_write_locked
  Module bzrlib.branch, line 517, in update_revisions
    self.fetch(other, stop_revision)
  Module <string>, line 4, in fetch_write_locked
  Module bzrlib.branch, line 279, in fetch
    pb=nested_pb)
  Module bzrlib.repository, line 975, in fetch
    find_ghosts=find_ghosts)
  Module <string>, line 4, in fetch_write_locked
  Module bzrlib.repository, line 2762, in fetch
    pb, find_ghosts)
  Module bzrlib.fetch, line 115, in __init__
    self.to_repository.commit_write_group()
  Module bzrlib.repository, line 942, in commit_write_group
    self._commit_write_group()
  Module bzrlib.repofmt.pack_repo, line 1794, in _commit_write_group
    return self._pack_collection._commit_write_group()
  Module bzrlib.repofmt.pack_repo, line 1643, in _commit_write_group
    if not self.autopack():
  Module bzrlib.repofmt.pack_repo, line 1209, in autopack
    self._execute_pack_operations(pack_operations)
  Module bzrlib.repofmt.pack_repo, line 1223, in _execute_pack_operations
    _packer_class(self, packs, '.autopack').pack()
  Module bzrlib.repofmt.pack_repo, line 585, in pack
    return self._create_pack_from_packs()
  Module bzrlib.repofmt.pack_repo, line 734, in _create_pack_from_packs
    self._check_references()
  Module bzrlib.repofmt.pack_repo, line 696, in _check_references
    missing_file_id)
RevisionNotPresent: Revision {<email address hidden>} not present in "source-20060927072757-wtb45rxgrsx81hqv-1".

Caused the following warning:

bzrlib/lockable_files.py:116: UserWarning: LockableFiles(<TransportElided url=/~launchpad-pqm/launchpad/devel/.bzr/repository>) was gc'd while locked
  warnings.warn("%r was gc'd while locked" % self)

lifeless has provided the following patch:
=== modified file 'bzrlib/repository.py'
--- bzrlib/repository.py 2008-10-09 04:50:37 +0000
+++ bzrlib/repository.py 2008-10-09 06:37:21 +0000
@@ -1027,9 +1027,12 @@
             self.control_files._lock_mode == 'w'):
             if self._write_group is not None:
                 self.abort_write_group()
- self.control_files.unlock()
+ self._unlock()
                 raise errors.BzrError(
                      'Must end write groups before releasing write locks.')
+ self._unlock()
+
+ def _unlock(self):
         self.control_files.unlock()
         for repo in self._fallback_repositories:
             repo.unlock()